The present invention relates to a power transmission apparatus applicable for vehicles having a differential mechanism for dividing torque between two output shafts. In particular, the present invention relates to an apparatus provided with a power takeoff unit with torque bias capable center differential construction for controlling torque transmission between front and rear axles in a four-wheel drive vehicle.
The performance advantages of four-wheel vehicle drive systems are well-recognized. Improved vehicle stability while traversing rain-soaked or ice and snow-covered highways, handling and control on gravel or uneven pavement and simply maintaining traction in off-road situations are all readily acknowledged benefits. Concomitant though less desirable characteristics of four-wheel drive systems relate to increased vehicle weight and increased drive-line friction which result in reduced gas mileage. Such increased drive-line friction results from the increased number of driven components and is especially significant in full-time four wheel drive systems.
Four-wheel drive vehicles having a transfer case in the drive-line for distributing power to the front and rear drive axles are known in the art. In such vehicles, the transfer case is usually provided with two or more output shafts which are driven by a main or input shaft. The driven shafts may be referred to as output drive shafts since they are used to drive the vehicle road wheels through drive axles. Some differential in the speed between the shafts is necessary to permit different rotational speeds of the driving wheels to accommodate vehicle steering. As a vehicle negotiates a corner, the front wheels traverse paths of longer average radius and length than the rear wheels. Therefore, there is a need for allowing speed differences between the front and rear axles to prevent the rear wheels from spinning. Additionally, the outer wheels traverse paths of longer average radius and length than the inner wheels. For example, when a car is steered into a 90-degree turn to the right and the inner wheel turns on a 30-foot radius, the inner wheel travels about 46 feet. The outer wheel, being on average nearly 5 feet from the inner wheel, travels nearly 58 feet. Hence, the outer wheel must rotate more rapidly during a turn. Without some means for allowing the drive wheels to rotate at different speeds, the wheels would skid when the car was turning. This would result in little control during turns and in excessive tire wear.
Furthermore, until a wheel skid occurs due to the common drive between the front and rear drive lines, the drive shafts will wind in opposite directions until a force produced by the stored (wind-up) torque in the drive""s shaft exceeds the frictional forces acting on the tires, the tires momentarily lose frictional contact, the drive lines unwind and the vehicle hops. Such operating conditions are both unacceptable to design engineers and unsettling to drivers. Installing a conventional differential assembly between the two drive lines such that they are capable of rotation at slightly different speeds solved the wind-up problem.
In some applications, a bevel gear differential, which evenly splits the torque between the drive axles, is used in the transfer case to drive the front and rear axles at all times, while allowing relative rotation between the axles to accommodate steering geometry. The use of a gear differential in a drive train has at least one drawback. That is, if any road wheel of the vehicle is on a low traction surface, the various axle and transfer case differentials allow that wheel to turn freely. As such, little power or torque is delivered to the remaining wheels.
To minimize wheel slippage, the transfer case differential is sometimes equipped with a manually operated lock-up mechanism. Such a mechanism is operated in either a locked or unlocked condition. When locked, such a mechanism connects the front and rear drive shafts together and positively drives them both at the same rotational speed. Such a locking mechanism does not allow any differentiation between front and rear drive axle turning speeds, thus leading to the previously noted problems.
Several systems have been devised to shift torque between the front and rear wheels. With such systems the clutch is aligned coaxially and serially in a linear torque path with other components of the system. A serious drawback of such designs is that there is an limit on the torque bias capability because of packaging constraints on the size of the clutch plates. In a typical vehicle there are axial packaging constraints from the engine mount, packaging constraints from the steering rack and diametrical packaging constraints from a front cross member and the engine oil pan. Thus, a need remains for a system wherein torque between front and rear drives may be shifted while taking full advantage of the packaging constraints in the vehicle.
Conventionally, a power transmission system capable of shifting torque is disclosed in U.S. Pat. No. 5,017,183, in which a differential gear mechanism coupled between two output shafts uses a planetary gear mechanism. U.S. Pat. No. 4,989,686 discloses a control system for a full time four-wheel drive having a transfer case including a planetary gear differential and an electromagnetically actuated clutch assembly for biasing torque between the front and rear drive wheels in response to signals for the control system. U.S. Pat. No. 4,718,303 discloses a four-wheel drive transfer case having a three element planetary gear differential connected to the output shaft of a transmission and having two output shafts which are driven by the differential in a predetermined timed relationship.
A power takeoff unit with torque bias capable center differential construction for four-wheel drive applications has been invented which overcomes many of the foregoing problems. In one aspect, the invention is a power takeoff unit for a four-wheel drive vehicle including a planetary gear arrangement for a planetary center differential having an axis of rotation. The planetary gear arrangement is connectable to an output shaft of a transmission. The gear arrangement includes a cage and at least three nested gear components for operably interconnecting the output shaft with both a front and rear wheel drive output shaft for full time drive to both output shafts. Input torque to the power takeoff unit is normally biased to both output shafts in a predetermined ratio. An actuatable friction clutch is aligned on the axis of rotation of the planetary gear arrangement. The clutch has an actuating mechanism and frictionally engagable members for interconnecting two of the planetary gear components in a manner such that the torque bias to the front and rear wheel drive output shafts is controlled by a torque level established between the frictionally engagable members of the clutch.
In another aspect, the invention is an automotive power takeoff unit for four-wheel drive vehicle having an electronically responsive biasing clutch that surrounds a front axle differential. The clutch is connected to a ring gear of said planetary gear arrangement and is grounded to a hollow shaft with an expanded bell to bias torque between the other two shafts and has clutch plates having an inner diameter and an outer diameter.
In yet another aspect, the invention is a method of splitting torque between the front wheels and rear wheels in a four wheel drive vehicle which includes providing an input shaft having a predetermined torque that is attached to a power takeoff unit. The power takeoff unit has a biasing clutch with a plurality of clutch plates having an inner diameter and an outer diameter. The clutch plates have splines on the inner diameter which engage the ring gear and splines on the outer diameter which engage the planetary carrier. The biasing clutch is aligned in a non-linear torque path on said axis of rotation. The method further includes the step of activating and deactivating the biasing clutch to distribute a predetermined torque split to the front and rear wheels.
The invention provides the foregoing and other features, and the advantages of the invention will become further apparent from the following detailed description of the presently preferred embodiments, read in conjunction with the accompanying drawings. The detailed description and drawings are merely illustrative of the invention and do not limit the scope of the invention, which is defined by the appended claims and equivalents thereof.